After that God has given you a promise, certain occasion(s) may show up that appears the way to the fulfillment of the promise. If you assumptively act on it without going back to God to get clear terms definition of the promise, you may end up taking a wrong decision, and it’s costly.
A Case Study: God responded to Abraham (still Abram then) that Eliezer wasn’t going to be his heir (Gen 15:4), and because he was in a tight condition of mind as per whether the promised son was going to be fulfilled through his wife considering her biological condition, as both being barren and at menopause, when Sarah (still Sarai then) his wife advised him to go in with Hagar if she could have kids by the means, I suppose Abraham quickly considered that the advice could be from God to fulfill the promise, so he sublet (Gen 16:2-3). He did so assumptively – not praying to find out from God if that was His mind. The occasions that followed this event weren’t palatable. We should learn from this.